Incompatibility of lmodern and fourier?

Question

Using the lmodern package, I had the typical font size issues in math mode: for example the sums were not large enough in some of my equations. To solve this I followed egreg's answer to the following question: sum symbol in tikzposter too small

The solution works for the sums but now my brackets are not scaled correctly anymore. Making a minimal working example I noticed that the issue is related to the fourier package. So I suppose there are incompatibilities somewhere. I know fourier has incompatibilities with other math fonts packages such as mathptmx, in my case it might be related to lmodern. Is it possible to use fourier and lmodern and avoid the scaling issues I have with my equations?

MWE:

\documentclass[a4paper, 12pt, twoside,openright]{report}
\usepackage{lmodern}
\usepackage[upright]{fourier}

% declare `cmex` to be arbitrary scalable
\DeclareFontShape{OMX}{cmex}{m}{n}{
  <-7.5> cmex7
  <7.5-8.5> cmex8
  <8.5-9.5> cmex9
  <9.5-> cmex10
}{}

\SetSymbolFont{largesymbols}{normal}{OMX}{cmex}{m}{n}
\SetSymbolFont{largesymbols}{bold}  {OMX}{cmex}{m}{n}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{bm}

\begin{document}
    \begin{equation}
    U_{qq} = \sum_{i < j \leq N_m} q_i q_j \left[ \sideset{}{'}\sum_{\bm{n}} \frac{\text{erfc}(\alpha \lvert\bm{r}_{ij} + L\bm{n}\rvert)}{\lvert\bm{r}_{ij} + L\bm{n}\rvert} + \frac{1}{\pi L} \sum_{\bm{n} \not= 0} \frac{1}{n^2} \exp(- \frac{\pi^2 n^2}{\alpha^2 L^2} + \frac{2 \pi i}{L} \bm{n} \cdot \bm{r}_{ij}) \right]
    \end{equation}
\end{document}

Result with fourier:

Result without fourier:

Answer 1

If you do \SetSymbolFont{largesymbols}{normal}{OMX}{cmex}{m}{n} after loading fourier, you're overriding the large symbol font and the result will be a horrible mixture.

Loading lmodern is useless if fourier is loaded too after it. Besides, fourier doesn't have the same problem as lmodern about non scalability of large symbols, because the font it provides is fully scalable at the outset.

Here's an example.

\documentclass[a4paper, 12pt, twoside,openright]{report}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{bm}
\usepackage[upright]{fourier}

\begin{document}

\begin{equation}
U_{qq} = \sum_{i < j \leq N_m} q_i q_j \left[ \sideset{}{'}\sum_{\bm{n}} \frac{\text{erfc}(\alpha \lvert\bm{r}_{ij} + L\bm{n}\rvert)}{\lvert\bm{r}_{ij} + L\bm{n}\rvert} + \frac{1}{\pi L} \sum_{\bm{n} \not= 0} \frac{1}{n^2} \exp(- \frac{\pi^2 n^2}{\alpha^2 L^2} + \frac{2 \pi i}{L} \bm{n} \cdot \bm{r}_{ij}) \right]
\end{equation}

$\sum\displaystyle\sum${\Huge$\sum\displaystyle\sum$}

\end{document}

As you see, the font scales well with size. It's only that the large summation symbol is, by designer's choice, smaller than the counterpart in Computer Modern, in which large symbols are, by default, very large.

It's not really difficult to grow the summation symbol larger in display style, but I can't recommend it.

\documentclass[a4paper, 12pt, twoside,openright]{report}

\usepackage[upright]{fourier}

\DeclareFontFamily{FMX}{futm-extra}{}
\DeclareFontShape{FMX}{futm-extra}{m}{n}{
   <-> s * [1.2] fourier-mex
}{}

\DeclareSymbolFont{extralargesymbols}{FMX}{futm-extra}{m}{n}
\DeclareMathSymbol{\sum}{\mathop}{extralargesymbols}{"50}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{bm}

\DeclareMathOperator{\erfc}{erfc}

\begin{document}

\begin{equation}
U_{qq} = \sum_{i < j \leq N_m} q_i q_j 
  \biggl[ \sideset{}{'}\sum_{\bm{n}} 
    \frac{\erfc(\alpha \lvert\bm{r}_{ij} + L\bm{n}\rvert)}
         {\lvert\bm{r}_{ij} + L\bm{n}\rvert} + \frac{1}{\pi L}
    \sum_{\bm{n} \not= 0} \frac{1}{n^2} \exp(- \frac{\pi^2 n^2}{\alpha^2 L^2}
      + \frac{2 \pi i}{L} \bm{n} \cdot \bm{r}_{ij})
  \biggr]
\end{equation}

\end{document}

I used \biggl and \biggr rather than \left and \right. More important is the definition of \erfc, because \text is wrong (more acceptable would be \mathrm, but see What's the difference between \mathrm and \operatorname?)

Answer 2

Here is one more example of the problems that arise when mixing lmodern and fourier. (It depends on the order in which they are loaded.)

\documentclass{article}
%\usepackage{lmodern,fourier} %plus symbol displays
\usepackage{fourier,lmodern}  %plus symbols does not display
\begin{document}
$a+b$
\end{document}